1. Field of the Invention
The present invention relates generally to the field of electric power, and more particularly to an anti-islanding device and method for inverters connected to a utility grid.
2. Description of the Related Art
FIG. 1 shows an example of a connection of an inverter 10 to a utility grid 12 and a local load 13. When circuit breaker CB1 is closed and circuit breaker CB2 is open, the load 13 is completely supplied by the inverter 10. When circuit breaker CB2 is closed and circuit breaker CB1 is open, the load 13 is supplied by the grid 12. When both circuit breakers are closed, the load 13 can get power from both the inverter 10 and the grid 12 depending upon the power output of the inverter 10 and the capacity of the load 13 itself. If the output power of the inverter 10 is greater than the capacity of the load 13, the remaining power from the inverter 10 flows to the grid 12 and vice versa. When the power of the inverter 10 matches exactly with the local load 13, the load 13 receives all power from the inverter 10 and the grid 12 simply remains floating. If circuit breaker CB2 is opened, no perturbation in voltage or frequency is observed, because the power balance has not been disturbed. Such a situation where the load 13 remains energized from the inverter 10 is called islanding.
When an inverter is connected to the grid, it is necessary to match the frequency as well as the voltage amplitude with the grid. The inverter uses the grid as the reference and generates an output voltage that is synchronized with the grid. If the grid becomes disconnected, the inverter will continue to supply power if the output power of the inverter matches with the local load demand on the grid since the inverter does not see any change in frequency or voltage. Such a condition is known as islanding, which constitutes a significant issue having substantial safety and performance implications.
For example, islanding results in a degradation of the quality of electricity supplied to the customer during the islanding period due to a lack of utility control. Uncontrolled frequency and voltage excursion can damage customer equipment. Further, if the grid disconnection is the result of a transient fault in the system, interrupting devices will try to re-close the grid connection after a few cycles (typically 12 to 15 cycles). Re-closing can potentially damage the inverter since the voltages in the island are not necessarily synchronized with the grid. When the grid is reconnected, it can have a different phase angle with respect to the islanded voltage, which can cause a large over-current that can damage the inverter already in the system and islanded with the load.
Islanding raises safety implications. For example, when the grid is disconnected to perform maintenance work on its power transmission lines, workers assume that the line is dead and safe to work on. However, the inverters continue supplying power and maintaining the voltage.
In order to address these concerns, an IEEE standard was developed for utility interconnection of PV (photovoltaic) systems, and an Underwriters Laboratories safety standard for photovoltaic inverters was also developed. Both of these standards are essentially the same regarding anti-islanding requirements. There is yet another IEEE standard draft underway to address the issues related to interconnecting distributed resources to the utility grid, which refers to the existing IEEE for utility interconnection of PV systems for anti-islanding requirements. In addressing the islanding issue, an aspect of the proposed standards relates to requiring the inverter to be able to detect the loss of the grid and disconnect as soon as the loss is detected. The standards require the inverter to disconnect within two seconds if the load matches within one percent. These requirements present a challenge to the industry.
Past approaches to anti-islanding involved, for example, either using extra signals, such as low frequency pulses or voltage harmonics, or introducing an explicit positive voltage or frequency feedback technique to detect the grid loss. These methods have serious drawbacks. Periodically injecting a signal can interfere with the grid or the load. For example, the periodic injected signals can sometimes cause resonance. In some instances, the injected signals can cancel each other, rendering the technique useless when multiple inverters are connected together. The frequency or voltage drifting technique uses explicit positive feedback. The change in frequency/voltage is measured and positive feedback is used to drift the frequency/voltage further away. This method is very sensitive to the gain selected for the positive feedback. Too small of a gain can make the system ineffective, resulting in the failure to detect the islanding within the required time frame, while too large of a gain can potentially drive the system unstable. Furthermore, if the load matches very closely with the inverter output, the change in frequency or the voltage may not be detectable. A non-detection zone (NDZ) exists in frequency or voltage drift techniques without active perturbation.
Approaches to deal with the islanding problem arose initially when the problem was first realized with solar power units having inverters which convert solar power into alternating current (AC) power, since solar power units, which have relatively small capacity, were the first to be connected to the grid. Different techniques, most of which are not standard, were used to address the islanding problem with solar power inverters. Some of the approaches use, for example, the injected signal method whereby a spike in voltage is periodically injected. If the grid is connected, the grid clamps the voltage and will not allow it to go beyond a certain point. As soon as the grid is lost, the spike shows up in the output voltage. All such approaches have problems with performance. For example, the injection of a regular periodic signal generates considerable harmonics/sub-harmonics in the system, which can interact with a load. Loads, such as a motor load, typically cannot tolerate very low frequency sub-harmonics. For example, 5 Hz or 10 Hz signals can cause problems in electric motors. In addition, only five percent total harmonic distortion is allowed in voltages, and introduction of the spike in the system may exceed this limit. Further, injected noise can resonate in filters in the system and cause an excessive current.
The approach in which a signal is injected and harmonics are detected in the output when the grid is not electrically present is referred to as an active anti-islanding method. Another approach is called the passive method. This method depends upon the change in voltage and frequency during the islanding condition. If the load mismatches during the loss of grid, then if real power is not matching exactly, there will be a small jump in voltage. Assume, for example, that the inverter is outputting 40 kilowatts to the grid and there is a load of 50 kilowatts in the grid. If the grid was previously supplying 10 kilowatts and becomes disconnected, all 50 kilowatts must come from the inverter. Thus, when the inverter power output increases, this will cause the voltage to drop, the change in voltage will be detected, and there is a positive feedback which accelerates the voltage drop further. If the grid was connected, the grid would have supported it, but if the grid is down, the voltage will start dropping, and that drop can be detected.
The passive method with a voltage drift positive feedback works if there is an imbalance in real power, but if there is an imbalance in reactive power, the current will not be in phase with voltage. Therefore, if there is a small power factor difference, for example, because the load is consuming slightly more reactive power than the inverter is supplying, and the grid will have to supply the difference, when the grid is disconnected, the inverter must supply that, so there is a sudden jump in phase angle between voltage and current. That appears as a frequency jump that can be detected. Whenever there is a sufficient change in frequency, positive feedback steps in and it is amplified further. It is obvious from the above description that the passive technique of voltage and frequency drift will not work if the mismatch in load is not big enough to generate the detectable change in frequency or voltage. The load mismatch within which the islanding is not detected is known as the non-detection zone (NDZ). Because islanding has safety implications, the standards require that islanding must be checked within one percent load matching. Due to the inherent measurement errors, one percent mismatch in load may fall into the NDZ and may not produce any measurable changes in the voltage and frequency.
The prior art techniques need either very accurate voltage and frequency measurements to introduce a positive feedback to drift the voltage or the frequency of the inverter or require the injection of some harmonics in the voltage signal to detect the islanding condition. The positive feedback technique with a very sensitive frequency and voltage measurement scheme with a higher gain can potentially cause instability and still does not guarantee the islanding detection for a perfect load match condition. The injection of voltage harmonics on the other hand can excite some resonance modes in the grid/load. This also affects the grid voltage quality.